<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    .outer{
      width: 400px;
      width: 400px;
      background-color: skyblue;
      border: 1px solid black;
    }
    .box{
      width: 100px;
      height: 100px;
      background-color: gray;
      margin: 10px;
    }
    .box1{
      float: right;
    }
  </style>
  <title>浮动小练习1</title>
</head>
<body>
  <div class="outer">
    <div class="box box1">1</div>
    <div class="box box2">2</div>
    <div class="box box3">3</div>
  </div>
  
</body>
</html>